U.S. Pat. No. 3,536,485, Roth et al is illustrative of a number of references which show a photoconductive drum in which the photoconductive surface is part of a flexible web. The web is first formed into an endless loop. The loop is mounted on a mandrel which has a natural size larger than the interior of the loop but which is contracted for purposes of mounting. The mandrel is allowed to expand to hold the loop and form a photoconductive drum.
Drums formed in this manner are less expensive than the traditional photoconductive drum in which the photoconductive layers are coated on the exterior of a continuous cylinder. The loops can be formed by traditional web coating and finishing operations in which extremely high quality is obtainable at low cost. When the photoconductive surface has worn out, the loop is replaced. This has an obvious advantage over reconditioning a coated drum which requires grinding and/or solvent treating the sensitive surface, finishing and recoating.
Photoconductive loops are cut from a larger web. They include a suitable support, for example, polyester, with the necessary electrophotographic layers coated thereon, for example, a conductive layer and one or more photoconductive and other layers that make up a modern photoconductive plate or web. After cutting to the desired size the loops are formed by connecting two opposite ends together at a seam, which may be ultrasonically formed. The seaming process itself is extremely well developed. However, at present, it is not possible to consistently eliminate meaningful amounts of conicalness from the final loop. That is, in general, the circumference of the loop at one end will be slightly different from the circumference at the other end. The loop, rather than being a perfect cylinder is slightly conical in shape.
Some conicalness in the final drum can be absorbed in the system by mounting components against the drum. That is, if the left side of a development station is maintained the same distance from the drum that the right side of the development station is maintained, the conicalness in the drum itself will not show up as an image defect in the final print or copy. However, if the mandrel for a loop is a perfect cylinder, and the loop itself is conical there will necessarily be some looseness of the loop at the larger circumference end. Such looseness permits dirt to get under the loop, which can cause image defects of a far more serious nature than conicalness of the drum itself. The looser edge acts differently to hard and soft backed stations than does the tight edge. In color systems, image registration can vary between the loose and tight edge.